Percentage Calculator

A percentage expresses one number as a fraction of another, scaled to a hundred. This tool answers the three everyday percentage questions — "what is X percent of Y", "X is what percent of Y", and "what is the percent change from X to Y" — live as you type. No input leaves your browser.

How to use

Pick one of the three calculation modes, enter the two numbers it asks for, and read the result.

1. What is X% of Y? — enter the percentage and the base value. For a 20% discount on 150 you get 30. Useful for sales tax, tips, interest on savings, or VAT on an invoice.
2. X is what percent of Y? — enter the part and the whole. If you scored 25 out of 200, you reached 12.5%. Useful for grades, completion rates, budgets, and survey results.
3. Percent change from X to Y — enter the starting and the final value. Going from 80 to 100 is a 25% increase; from 100 to 80 is a 20% decrease (the sign flips automatically). Useful for price rises, salary changes, population growth, and crypto or stock performance.

The result is shown with a fixed label that repeats your question with the actual numbers ("20% of 150"), so the answer always has context. When a question is mathematically undefined — dividing by a whole or by a starting value of zero — the tool says so explicitly instead of showing a confusing figure.

A quick guide to the arithmetic

• "…% of …" means multiplying: 15% of 40 equals 0.15 × 40 = 6.
• "… is what % of …" means dividing: 6 is 15% of 40 because 6 ÷ 40 = 0.15.
• A percentage change is the difference divided by the old value: (new − old) ÷ old. If something grows from 40 to 46, that is (46 − 40) ÷ 40 = 15%.
• Percentages add and multiply non-intuitively: two consecutive 10% changes do not equal one 20% change (they equal 21%), which is why it is worth letting the tool do the third calculation instead of doing it in your head.

When to reach for this tool

Percentage math sits across shopping, budgeting, and growth tracking. Use mode three when comparing values that change over time — a salary from one year to the next, the price of an item before and after a sale, or a portfolio across a reporting period. Use mode two whenever you want a part of a whole as a fraction of 100, and mode one whenever you need to apply a rate to an amount. When you also need to judge whether one percentage is a big deal in context — say a 10% increase on a very small base — check the change mode, which always shows the sign and lets you sanity-check the magnitude against the base number.

Notes and limits

The calculator follows the standard interpretation of percentage as "per hundred". Results are rounded for display but computed from the full precision of your inputs. There is no hidden rounding mode and no currency or locale logic, so amounts are shown as plain numbers you can format yourself; percentages of zero and changes from zero are correctly reported as undefined. For compound situations such as annual percentage rates or margin calculations, apply the same operations repeatedly or consult a dedicated financial calculator.